Distribution of life-history stages of the salp Thalia democratica in shelf waters during a spring bloom

Swarms of the salp Thalia democratica periodically occur off southeast Australia following the austral spring bloom of phytoplankton. The present study aimed to determine the abundance and size/stage distribution of T. democratica and their relationship with copepods in 3 water types of the western Tasman Sea. Samples were taken from vertical net hauls along 4 cross-shelf transects spaced along 200 km of the New South Wales coast, from the East Australian Current (EAC) separation zone, around 32.5 degrees S, to off Sydney (34 degrees S). Temperature-salinity signatures grouped stations into 3 distinct water types: inner shelf water, EAC and upwelled water. Although common across all stations, T. democratica was significantly more abundant in inner shelf waters compared to both EAC and upwelled water. Analysis of population structure (aggregate buds, aggregate females, aggregate males, immature solitaries and mature solitaries) also identified higher proportions of reproductive aggregates and their offspring in inner shelf water. This salp population structure was significantly different in the EAC regions, characterised by a paucity of the solitary stages, higher temperatures and lower chlorophyll a concentrations. A weak negative correlation was identified between T. democratica and copepod abundance. In the present study, the maximum abundance of T. democratica was twice the highest globally recorded abundance and 10-fold greater than maximum abundances sampled from the continental shelf and slope waters off southeast Australia during the period from 1938 to 1942.